Let F=age of father and S=age of son. F=3S.
In 8 years' time they will be F+8 and S+8 years old and F+8=5/2(S+8), because 2 1/2=5/2, so, multiplying through by 2, 2F+16=5S+40; 2F-5S=24; substituting for F, 6S-5S=24; S=24 and F=3S=72. In 16 years' time, son will be 40 and father will be 88, so 88/40=11/5=2 1/11.